This is a revised proposal (R01AI150471-01) in response to PA 99-124. The goal of this project is a comprehensive characterization of Cryptosporidium parvum type 1, the most common agent of human cryptosporidiosis in the general population and in people with AIDS. Because until recently type 1 C. parvum could not be propagated in the laboratory, this subgroup is rarely studied and remains poorly characterized. As a consequence, phenotypic properties of calf-propagated type 2 oocysts are extrapolated to the whole species. In light of the significant public health importance of cryptosporidiosis caused by type 1 C. parvum, the emphasis of this proposal is twofold: 1) to extensively characterize C. parvum type 1 with respect to genotypic and phenotypic properties, as well as its life cycle and interaction with the host, and (2) to compare C. parvum type 1 and type 2 with the aim of gaining a better understanding of the species C. parvum and the disease caused by this parasite. The recent development of a gnotobiotic pig model suitable for propagation of type 1 and type 2 will facilitate the study of several type 1 isolates and the comparison of standardized C. parvum isolates of both types originating from the same host. The interaction of type 1 and type 2 isolates in mixed infections will also be studied in the pig model. Type 1 isolates included in this study will originate from chronic and acute infections and from different geographical areas. The study includes three specific aims: the first aim is a comprehensive analysis of ten type 1 C. parvum isolates. In the second aim the host-parasite interaction of type 1 and type 2 isolates will be compared using the pig model and cell culture models. The third aim investigates the extent of genetic variation and exchange within and between the two types of C. parvum to determine whether they belong to one or two species.